Abstract

Donor electron spins in semiconductors make exceptional quantum bits because
of their long coherence times and compatibility with industrial fabrication
techniques. Despite many advances in donor-based qubit technology, it remains
difficult to selectively manipulate single donor electron spins. Here, we show
that by replacing the prevailing semiconductor host material (silicon) with
germanium, donor electron spin qubits can be electrically tuned by more than an
ensemble linewidth, making them compatible with gate addressable quantum
computing architectures. Using X-band pulsed electron spin resonance, we
measured the Stark effect for donor electron spins in germanium. We resolved
both spin-orbit and hyperfine Stark shifts and found that at 0.4 T, the
spin-orbit Stark shift dominates. The spin-orbit Stark shift is highly
anisotropic, depending on the electric field orientation relative to the
crystal axes and external magnetic field. When the Stark shift is maximized,
the spin-orbit Stark parameter is four orders of magnitude larger than in
silicon. At select orientations a hyperfine Stark effect was also resolved and
is an order of magnitude larger than in silicon. We report the Stark parameters
for $^{75}$As and $^{31}$P donor electrons and compare them to the available
theory. Our data reveal that $^{31}$P donors in germanium can be tuned by at
least four times the ensemble linewidth making germanium an appealing new host
material for spin qubits that offers major advantages over silicon.

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